1. Field of the Invention
This invention relates to a method and apparatus for recording/reproducing data on a rotatable record medium and, more particularly, to a technique by which image data is recorded in data-compressed form but, nevertheless, can be reproduced when the medium is scanned in a special mode, such as a high speed search mode, to permit a viewable display of the recorded images while that medium is scanned.
2. Description of the Prior Art
The recording of image information on a record medium has long been known. Magnetic recording permits the re-use of a magnetic medium to record and reproduce image information several times. For example, image information is recorded in slant tracks on a magnetic tape or in successive turns on a magnetic disk, such as the successive turns of a spiral track or in concentric circular tracks on the disk. Image information also has been recorded on video disks, such as laser disks in which image data is represented by pits and lands. Recently, it has been proposed that image information in the form of video signals be recorded on an optically readable disk conforming generally in size and physical characteristics to the so-called audio "compact disk", conventionally referred to as a "CD". Such information may be recorded in successive turns of a single spiral track or in consecutive circular tracks.
A compact disk, on which information may be thought of as being permanently recorded, may be "read" several times. Such a disk functions as a memory for recorded digital information and often is referred to as a "CD-ROM". Compact disks and CD-ROM's generally conform to "standards" which tend to limit the amount of information that can be recorded thereon. Such standards establish, inter alia, the rotary speed of the disk and the number and density of the tracks that may be recorded thereon. When digital video information is recorded, as by digitizing a conventional videosignal (e.g. an NTSC signal), the CD-ROM conforming to such standards exhibits a recording capacity on the order of about one hour. Thus, a video program whose duration does not exceed approximately one hour may be recorded and played back from such a CD-ROM.
It is appreciated that a larger quantity of data may be recorded on a CD-ROM if that data is subjected to data compression techniques. For example, the inherent redundancy in video information suggests that video signals are favorably susceptible to data compression. An adaptive process for the coding and decoding of a sequence of pictures by discrete cosine transformation has been proposed in U.S. Pat. No. 4,707,738. As described in this patent, video signals may be data compressed by producing "inter-picture" and "intra-picture" representations of image information. The inter-picture information, or code, represents the differences between two successive pictures, or frames. This difference information constitutes a substantial compression of data, although a video picture cannot be reproduced from a given inter-picture code signal without a reference picture or frame to which the difference information may be combined to reconstitute the encoded picture. However, the intra-picture information represents substantially all of the information of a given frame and, thus, constitutes a rather small amount of data compression.
Using the adaptive coding process of the aforementioned patent, intraframe code signals and interframe code signals may be recorded on a CD-ROM to extend the effective capacity of that CD-ROM for recording video signals. This technique is particularly advantageous when highly animated image data is encoded. Thus, by using intraframe and interframe code signals, image data which varies rapidly over several frames may be data compressed. Such encoding of image information into intraframe and interframe code signals is particularly adapted for compressing the large amount of information present in a high density television signal.
Typically, a first image frame is encoded as an intraframe code signal and then, several frames later, another image frame is encoded as an intraframe code signal. Those image frames which extend between the successive intraframe encoded frames are encoded as interframe code signals. When these intraframe and interframe code signals are recorded on a CD-ROM, such as in successive turns of a spiral track, an intraframe code signal may be recorded in one turn but the next intraframe code signal may not be recorded for several turns. This may not present any difficulties during normal video playback modes because the several successive interframe code signals which are reproduced are combined with the information derived from the previously reproduced intraframe code signal to recover whole frames of image data for display as a conventional television picture having excellent fidelity. Indeed, several turns of merely interframe code signals may be reproduced without any noticeable deterioration in the quality of the displayed picture.
However, when one intraframe code signal followed by several turns of interframe code signals are recorded, the television picture reproduced therefrom during special reproducing modes, such as a high speed search mode, may not be acceptable. In a high speed search mode, the playback head is moved radially across the record medium at a relatively high speed. Hence, successive interframe code signals are not reproduced. Rather, the radial movement of the head during this "track jumping" operation reproduces selective, individual interframe code signals that may vary greatly from the previous intraframe code signal that was played back. Hence, image frames which are derived from these individual interframe code signals may exhibit less than acceptable quality during special reproducing modes. Therefore, if image information is to be recorded on a rotatable medium, such as a CD-ROM, data compression by way of interframe coding may not permit satisfactory high speed searching.